In recent years, there are color electrophotographic apparatuses that have a fixing device with a function of applying gloss to an image formed on a sheet having a high degree of gloss, so that the glossiness (degree of gloss) of the image is similar to that of the background.
A method of increasing the degree of gloss is typically performed by conveying the sheet at a linear speed of less than or equal to half of a regular speed and increasing the fixing heat amount. Accordingly, the toner image is further gelatinized compared to a regular process, and the toner particles are fluidized, so that gloss is applied to the image.
Furthermore, at the nip part in the fixing device used for applying heat and pressure to the sheet, the pressure is increased for leveling the surface of the image, thereby applying a high degree of gloss to the image. The fixing device is to be configured to generate high nip pressure so that a high degree of gloss can be applied to images. For the purpose of increasing the productivity with the use of such a fixing device, the size of the fixing device may be increased. However, when the fixing device having such a configuration is used for applying gloss, the amount of heat applied to the sheet is increased by decreasing the linear speed of the sheet, and therefore the productivity is significantly reduced.
Conventionally, a technology for controlling the glossiness has been proposed, as described in patent document 1. That is, plural fixing devices are provided, and the glossiness is controlled according to the number of fixing devices through which the sheet is passed. In this technology, plural fixing devices are provided, and therefore two conveying paths (sheet path lines) need to be provided. Specifically, the number of fixing devices to be used can be selected by providing two conveying paths, according to whether gloss is to be applied or not (gloss is not applied when plain paper is used as the sheet).
Patent document 2 proposes a method of switching between applying/not applying gloss according to need, by separating rollers in a first fixing procedure and a second fixing procedure. However, the toner may not be completely fixed on the sheet that is conveyed between the first fixing procedure and the second fixing procedure. Thus, the configuration is made in consideration of preventing an unfixed image from being damaged. With such a configuration, it is difficult to completely correct the condition of the conveyed sheet with the use of a common guiding plate connecting the first and second fixing procedures. Therefore, paper jams have occurred.
In patent document 3, the roller used for the second fixing procedure is heated to a lower temperature than the sheet insertion temperature (temperature when the sheet is inserted), for the purpose of simplifying the second fixing procedure. However, when the sheet is passed through the first and second fixing procedures with a guide plate, the heat (surface temperature) of the sheet and the surface layer of the toner is taken by the guide plate. Therefore, the intended purpose of applying gloss cannot be attained.
Patent Document 1: Japanese Laid-Open Patent Application No. S63-192068
Patent Document 2: Japanese Laid-Open Patent Application No. 2003-167459
Patent Document 3: Japanese Laid-Open Patent Application No. 2004-139040
Accordingly, there is a need for a compact, low-price image forming apparatus providing an option for selecting a mode for applying gloss to an image on a sheet (gloss applying mode) or a mode for not applying gloss to an image on a sheet (non-gloss-applying mode), with which gloss can be appropriately applied in the gloss applying mode, and the sheet can be conveyed in a stable manner in either of the modes.